In various use applications such as cloths, bags, shoes, packing materials and the like, a buckle which includes a plug and a socket to connect corded members, such as belts and the like, to each other has been known. The plug includes a base having an attachment for attaching a corded member, and a pair of left and right arms extending from one end of the base. The socket includes a cylindrical main body having a hollow part to accommodate therein the arms of the plug. Openings to engage the arms of the plug are provided on both sides of this main body. It is called as a side engagement buckle (side release buckle) since the plug and the socket are detachably engaged at the openings formed on both sides of the main body of the socket.
As an example of the side release buckle, for example, U.S. Pat. No. 5,794,316 (Patent Document 1) discloses an example in which V-shaped stop members are provided within a socket member, and V-shaped latch surfaces are placed on the arms of the plug member. The plug member and the socket member are detachably connected to each other as the arms of the plug member is inserted into the socket member and the V-shaped stop members and the V-shaped latch surfaces are engaged with each other on both sides of the socket member.
Japanese Utility Model Registration No. 2533866 (Patent Document 2) discloses an example of a side release buckle of a front and back engagement type, which includes a buckle main body and an insertion body. Since engagement units placed on front and back sides of a tip of a leg of the insertion body and a second engagement unit placed on a central side of the leg are engaged with or disengaged from engagement units placed on a buckle main body side, respectively, the buckle main body and the insertion body are attached to or detached from each other.